How does spring singleton beans serve concurrent requests (#11987)

* add service base structure
- controllers & services & sanity check tests

* finalize code

* reformat continues lines with 2 spaces

* reformat continues lines with 2 spaces

spring-core-5/src/main/java/com/baeldung/concurrentrequest/ConcurrentRequestApplication.java

@@ -0,0 +1,11 @@
+package com.baeldung.concurrentrequest;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+@SpringBootApplication
+public class ConcurrentRequestApplication {
+    public static void main(String[] args) {
+        SpringApplication.run(ConcurrentRequestApplication.class, args);
+    }
+}

spring-core-5/src/main/java/com/baeldung/concurrentrequest/Product.java

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+package com.baeldung.concurrentrequest;
+
+public class Product {
+
+    private final int id;
+    private final String name;
+    private final Stock stock;
+
+    public Product(int id, String name, Stock stock) {
+        this.id = id;
+        this.name = name;
+        this.stock = stock;
+    }
+
+    public Stock getStock() {
+        return stock;
+    }
+
+    public int getId() {
+        return id;
+    }
+
+    public String getName() {
+        return name;
+    }
+}

spring-core-5/src/main/java/com/baeldung/concurrentrequest/ProductController.java

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+package com.baeldung.concurrentrequest;
+
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.PathVariable;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+@RestController()
+@RequestMapping("product")
+public class ProductController {
+
+    private final ProductService productService;
+
+    public ProductController(ProductService productService) {
+        this.productService = productService;
+    }
+
+    @GetMapping("/{id}")
+    public Product getProductDetails(@PathVariable("id") int productId) {
+        return productService.getProductById(productId)
+          .orElse(null);
+    }
+
+    @GetMapping("{id}/stock")
+    public Stock getProductStock(@PathVariable("id") int productId) {
+        return productService.getProductById(productId)
+          .map(Product::getStock)
+          .orElse(null);
+    }
+}

spring-core-5/src/main/java/com/baeldung/concurrentrequest/ProductService.java

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+package com.baeldung.concurrentrequest;
+
+import static java.lang.Thread.currentThread;
+import static java.util.Arrays.asList;
+
+import java.util.List;
+import java.util.Optional;
+
+import org.springframework.stereotype.Service;
+
+@Service
+public class ProductService {
+
+    // @formatter:off
+    private final static List<Product> productRepository = asList(
+        new Product(1, "Product 1", new Stock(100)),
+        new Product(2, "Product 2", new Stock(50))
+    );
+    // @formatter:on
+
+    public Optional<Product> getProductById(int id) {
+        Optional<Product> product = productRepository.stream()
+          .filter(p -> p.getId() == id)
+          .findFirst();
+        String productName = product.map(Product::getName)
+          .orElse(null);
+
+        System.out.printf("Thread: %s; bean instance: %s; product id: %s has the name: %s%n", currentThread().getName(), this, id, productName);
+
+        return product;
+    }
+}

spring-core-5/src/main/java/com/baeldung/concurrentrequest/Stock.java

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+package com.baeldung.concurrentrequest;
+
+public class Stock {
+    private final int inStockItems;
+
+    public Stock(int inStockItems) {
+        this.inStockItems = inStockItems;
+    }
+
+    public int getInStockItems() {
+        return inStockItems;
+    }
+}

spring-core-5/src/test/java/com/baeldung/concurrentrequest/ConcurrentRequestUnitTest.java

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+package com.baeldung.concurrentrequest;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
+
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.test.web.servlet.MockMvc;
+import org.springframework.test.web.servlet.ResultMatcher;
+
+@SpringBootTest
+@AutoConfigureMockMvc
+public class ConcurrentRequestUnitTest {
+
+    @Autowired
+    private MockMvc mockMvc;
+
+    @Autowired
+    private ProductController controller;
+
+    @Test
+    public void givenContextLoads_thenProductControllerIsAvailable() {
+        assertThat(controller).isNotNull();
+    }
+
+    @Test
+    public void givenMultipleCallsRunInParallel_thenAllCallsReturn200() throws Exception {
+        ExecutorService executor = Executors.newFixedThreadPool(2);
+
+        executor.submit(() -> performCall("/product/1", status().isOk()));
+        executor.submit(() -> performCall("/product/2/stock", status().isOk()));
+
+        if (!executor.awaitTermination(60, TimeUnit.SECONDS)) {
+            executor.shutdownNow();
+        }
+    }
+
+    private void performCall(String url, ResultMatcher expect) {
+        try {
+            this.mockMvc.perform(get(url))
+              .andExpect(expect);
+        } catch (Exception e) {
+            throw new RuntimeException(e);
+        }
+    }
+}